Thin film patterning is an essential technology that has enabled the miniaturization of many technologies in both the electronics and biomedical fields. Patterned thin films can be found in a variety of electronics applications including batteries, solar cells, transistors, microfluidics, patch antennas, and touch panels [Yagi, I., et al., “Direct observation of contact and channel resistance in pentacene four-terminal thin-film transistor patterned by laser ablation method,” Appl. Phys. Lett. 84 (2004) 813-815; Cho, G., et al., “Patterned Si thin film electrodes for enhancing structural stability,” Nanoscale Research Letters 7:20 (2012) 1-5; Tseng, S-F., et al., “Laser scribing of indium tin oxide (ITO) thin films deposited on various substrates for touch panels” (2010); Gecys, P., et al., “Scribing of Thin-film Solar Cells with Picosecond Laser Pulses” (2011); Ruthe, D., et al., “Etching of CuInSe2 thin films—comparison of femtosecond and picosecond laser ablation” (2005)]. The biomedical field has developed a great need for such thin electrode arrays as well, particularly for point-of-care applications. Concepts such as the “lab-on-chip,” implantable sensors, and ingestible probes rely on the biotech industry's ability to accurately and reproducibly create microscopic electrical conduits on increasingly thin substrates [Henderson, R. D., et al., “Lab-on-a-Chip device with laser-patterned polymer electrodes for high voltage application and contactless conductivity detection” Chem. Commun., 48 (2012) 9287-9289; Chin, C. D., et al., “Commercialization of microfluidic point-of-care diagnostics devices,” Lab Chip 12 (2012) 2118-2134]. Additionally, the use of organic, flexible substrates will allow implementation of such electrodes into a wider range of products for both industries that can provide users and researchers with highly adaptable geometries for consumer and industrial electronics. If devices are to progressively occupy less space and continually exhibit higher functionality, all while remaining relatively low cost, then industrial processes must match this demand by developing innovative ways of patterning deposited thin films.
Two of the most prevalent modes of patterning thin films are shadow masked deposition and photolithography [Glang, Reinhard, and Lawrence V. Gregor. “Generation of Patterns in Thin Films.” Handbook of Thin Film Technology, New York: McGraw-Hill, (1970) 7.1-7.66]. The former technique requires the creation of multiple yet expensive masks, which make changing designs, even slightly, both costly and time-consuming. The latter technique also uses masks for the purposes of patterning the area of exposure and incorporates various photoresists and chemical development steps leading to highly accurate features. In turn, the process is complicated and introduces the need for a number of extra environmental engineering controls to deal with such potentially harmful chemicals. Some of drawbacks can be mitigated by using directed energy techniques to scribe out the negative space of a film, precluding the need for masks or etching steps. Such processes can use focused e-beams or laser radiation coupled with a scanning system to reduce production time, however, these techniques are still primarily subtractive in nature and result in the vaporization of a great deal of material that cannot be recovered. Other techniques, including those that are additive in nature (e.g. ink jet printing) [Levy, D. H., et al., “Metal-oxide thin-film transistors patterned by printing,” Appl. Phys. Lett. 103 (2013) 043505], have their own drawbacks that must be overcome during implementation into current manufacturing processes. An alternative to current patterning techniques involves the use of dewetting to build patterned structures from an initially uniform target layer.
The shape of liquids on solid surfaces is dictated by the contact angle (FIG. 10). The latter is related to the materials' surface energy by the Young's Equation (Equation 1), where θc is the contact angle in thermodynamic equilibrium. Liquids that completely wet the solid correspond to θc=0°. Angles between 0° and 90° can be considered partially wetting, while angles greater than 90° are formed by non-wetting liquids. The wetting nature of a material can be described by the change in free energy with respect to the standard state, i.e. complete wetting of the solid by the liquid [Gentili, D., et al., “Applications of dewetting in micro and nanotechnology,” Chem. Soc. Rev. 41 (2012) 4430-4443]. This free energy is a combination of gravitational forces, intermolecular forces, and surface tension (Equation 2). Surface topography will affect the apparent contact angle and by consequence the measured surface energy; however, materials will exhibit surface energy values in general domains intrinsic to material classification. For example, metals tend to lie in the highest range of surface energy values, while materials such as polymers tend to have the lowest values. Interfacial force influences the dewetting of metallic films deposited on substrates such as glass or polymeric compounds and can be measured by the spreading coefficient S (Equation 3). A film will spontaneously dewet when the change in free energy S<0 and has a thickness less than a critical height. In turn, this critical height is a function of capillary length and equilibrium contact angle and provides the basis for determining energetic stability of the film with respect to the substrate [Sharma, A. and Ruckenstein, E. “Energetic Criteria for the Breakup of Liquid Films on Nonwetting Solid Surfaces,” Journal of Colloid and Interface Science. 137, 2 (1990) 433-445]. Studies on polymer films have shown that dewetting can occur by a number of mechanisms, including nucleation and growth of holes within the film and spontaneous spinodal dewetting [Stange, T. G., Evans, D. F., “Nucleation and Growth of Defects Leading to Dewetting of Thin Polymer Films” Langmuir 13 (1997) 4459-4465; Xie, R., et al., “Spinodal Dewetting of Thin Polymer Films,” Phys. Rev. Lett. 81, 6 (1998) 1251-1254]. Researchers have also initiated dewetting in films that would not do so independently. A number of mechanical methods have been employed, rather than relying on random impurities to serve as nucleation points [Pandit, A. B., and Davidson, J. F. “Hydrodynamics of the rupture of thin liquid films” J. Fluid Mech. 212, 11 (1990) 11-24; Redon, C., et al., “Dynamics of Dewetting” Phys. Rev. Lett. 66, 6 (1991) 715-718]. By introducing this external energetic input, some control is established over the dewetting geometry and progression of a film on non-wetting surfaces.
                                          γ            SL                    +                                    γ              LG                        ⁢            cos            ⁢                                                  ⁢                          θ              c                                      =                  γ          SG                                    (        1        )                                          Δ          ⁢                                          ⁢                      F            ⁡                          (              h              )                                      =                                            1              2                        ⁢            ρ            ⁢                                                  ⁢                          gh              2                                -          S          +                      A                          12              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              h                2                                                                        (        2        )                                S        =                              γ            SG                    -                      (                                          γ                SL                            +                              γ                LG                                      )                                              (        3        )            
The described invention presents a novel variation on induced dewetting which allows for rapid, directed patterning through the use of a high speed scanning laser. For example, by taking a relatively low-powered Yb doped fiber laser and scanning it across the surface of a metallic thin-film, targeted melting and dewetting of the target film can be achieved without damaging the underlying polymer substrate. There is some precedent for induced dewetting of metallic, thin film targets under a variety of non-scanning laser radiation conditions. In 1996, Bischof et. al. studied the dewetting modes of thin films after exposure from frequency doubled Nd:YAG radiation [Bischof, J., et al., “Dewetting Modes of Thin Metallic Films: Nucleation of Holes and Spinodal Dewetting,” Physical Review Letters (1996) 77 (8) 1536-1539]. The authors demonstrate that deposited Au, Cu, and Ni thin films, which are metastable in nature, will dewet upon induced melting and will do so by nucleation of holes or by a spontaneous, spinodal fashion. Other researchers have built upon this dewetting phenomenon to create self-assembled features on Co nano-films by using defocused laser light and pulses in the nanosecond regime [Favazza, C., et al., “Robust nanopatterning by laser-induced dewetting of metal nanofilms,” Nanotechnology 17 (2006) 4229-4234]. It was shown that characteristic length scales developed as functions of the number pulses used, which allowed some control over the final dimensions of the pattern. Splitting a laser source into two or three beams to create interference has enabled the creation of periodic patterns on a variety of thin-film species, including Bi, Ge, Ni, Au, Cu, and Ta. Changing the arrangement and number of beams allows even more control over pattern design; however, such techniques are very much limited to lines, hexagonal patterns, and others associated with intensity interference [Riedel, S., et al., “Nanostructuring of thin films by ns pulsed laser interference,” Appl Phys A 101, (2010) 309-312; Riedel, S., et al., “Pulsed Laser Interference Patterning of Metallic Thin Films” Acta Physica Polonica A 121, 2 (2012) 385-387]. Placing masks along the beam path, which are then focused down to a smaller scale, offered additional freedom in shape patterning similar to photoresist methods, but suffer from the same drawbacks [Kuznetsov, A. I., et al., “Nanostructuring of thin gold films by femtosecond lasers,” Appl Phys A 94, (2009) 221-230]. Pre-patterning of the thin film, followed by laser exposure imposes constraints on the acting surface tension in the melt and allowed a “directed assembly” of the dewetted material [Rack, P. D., et al., “Pulsed laser dewetting of patterned thin metal films: A means of directed assembly” Applied Physics Letters 92, 223108 (2008); Fowlkes, J. D., et al., “Self-Assembly versus Directed Assembly of nanoparticles via Pulsed Laser Induced Dewetting of Patterned Metal Films” Nano Letters 11 (2011), 2478-2485]. The techniques still require the creation of masks for electron lithography and the subsequent use of lift-off processes, which hinders the speed of implementation. In sharp contrast to the aforementioned techniques, the processing technique of the described invention relies on a number of system features, namely a high speed scanning system that affords the user a great deal of freedom in rapid direct scribe dewetting programs, while eliminating the need for costly lithographic masks and time-consuming pre-patterning steps.
Choice of system materials is crucial to the technique of the described invention as a number of attributes can affect both the dewetting behavior of the metal as well as its interaction with the laser radiation (Table 1). For example, bismuth as the deposited metal has a relatively low melting-point with a much higher vaporization-point, and a large liquid surface tension value. Additionally, bismuth absorbs a relatively large percentage of incident near-infrared (NIR) radiation, whereas other metals may reflect a much larger percentage of incident radiation at that wavelength. The combination of low melting temperature and higher absorption makes it particularly applicable to polymer substrates. Tin presents as another viable candidate, as it also has a low melting point, high vaporization point, high surface tension, and exhibits relatively high absorption of NIR light. However, lasers of shorter wavelengths can be utilized to enable a wide array of metals due to improved absorption characteristics. One specific phase of a material may preferentially couple with one laser wavelength over another and thus affect the ability to melt and dewet from a substrate without significant amounts of material loss. (PVD) of the target metal on the substrate, for example, enables extremely fast cooling rates and the kinetic stabilization of metal films and a metastable, uniform wetting of the substrate.
A suitable substrate would exhibit a lower surface energy than a metal, would be an insulator, and would also have reduced absorption of NIR light to avoid destruction during laser processing. Reduced absorption also allows “back-side” dewetting by transmission through the substrate. For example, both borosilicate glass and parylene-C (par-C) meet these criteria. Where glass is highly transparent to NIR light, the surface energy is relatively high compared to most polymers. Par-C boasts a surface energy comparable to polytetrafluoroethylene (PTFE) polymers, making it relatively easy for deposited metal to dewet from the surface. The polymer can be deposited upon a borosilicate glass slide, which provides the necessary rigidity during processing, but still allows subsequent removal from the substrate. By no means limiting, other substrates can also be utilized, in addition other substrates that have properties that initially seem unsuitable, such as other metals, can be enabled by a thin coating of the of the suitable substrate such as parylene on the surface. Physical vapor deposition
TABLE 1Relevant properties of target and substrate candidatesSurface EnergySolid DensityLiquid Density[mJ/m2]Absorption atMaterial[g/cm3][g/cm3]M.P. [° C.]SolidLiquid1000 nm [%]Parylene-C1.289—29019.6—<15Parylene-N1.11—42045—<10Borosilicate Glass2.23—821 (soften) 253.0—<3Polyimide (Kapton)1.42—400 (soften)53—<10PTFE (Teflon)2.2—32720—<10PVDF homopolymer1.78—17730.3——(Kynar)Bismuth9.7810.05271.5382378—Tin7.3656.99231.9351455446Zinc7.146.57419.55—81120Gallium5.916.09529.85—707—Antimony6.6976.53630.63—38445Aluminum2.72.375660.3241.291429Indium7.317.021499.85—559—Silver10.499.329621302930<5
The described invention provides a new method of patterning metallic thin films that overcomes the deficiencies of the current methods. Through the use of a focused laser deflected by a high-speed, galvanometer scanning system, a variety of fine metal patterns can be realized on a number of inorganic and organic substrates. This method exploits the metastable wetting characteristics of metallic thin films as deposited by physical vapor deposition or other techniques upon non-metallic substrates. Differences in surface energy and intermolecular forces between the target and the substrate provide a driving force for retraction of the thin film, while the thermal energy from the laser provides the energy needed to overcome the kinetic barrier. Electronically isolated feature sizes in the range of the tens of microns can be fabricated. During formation, material is displaced rather than ablated allowing controlled accumulation of the target material. This results in a user-determined increase of the metal feature thickness. The described invention provides for the creation of accurate and reproducible periodic structures, as well as complex designs. This technique provides an alternative to current thin film patterning techniques and introduces a new way of building out-of-plane structures in thickness from metallic thin films. This process easily lends itself to integration into existing industrial processes.